The present invention relates to resonators for high frequency electromagnetic oscillators. The most important characteristics of resonators of this type are its resonant frequencies f.sub.0 and its quality factor Q. The present invention is directed towards a resonator which is small in volume but exhibits a high quality factor.
Prior art resonators may be divided into two principal categories; open systems and shielded systems. Both classes of resonators are often used in frequency tuned circuits, for example, in the form of bandpass filters or band rejection filters. Open systems, such as Fabry-Perot resonators, microstrip resonators, and certain dielectric resonators, are useful in circuits with relatively low selectivity requirements while shielded systems such as different coaxial and cavity resonators, triplate resonators and yet other dielectric resonators, are useful in circuits requiring relatively high selectivities. Microstrip or triplate resonators are circuit elements employing the unsymmetric and symmetric stripline techniques, respectively. Typical constructions are the open half-wave resonator, the circular disk and the circular ring resonators.
Resonators which exhibit a high degree of reproducibility, small size, high consistency of performance and low cost are preferred. When designing such resonators it is desirable to avoid high galvanic losses which result in relatively low quality factors. With particular reference to microstrip resonators, it is desirable to minimize reflection and dielectric losses. Generally speaking, stripline bandpass filters exhibit relatively high filter losses and relatively low selectivity. For this reason, such filters are suited chiefly for circuit applications which place no special requirements on transmission quality.
Dielectric resonators are volume resonators and are used in stripline resonators as well as cavity resonators. Such resonators may take the form of discs, rings, cylinder or square blocks. Open resonators can be divided into three broad categories; one, two and three-dimensional open resonators. In order for an open resonator to oscillate the electromagnetic field must propagate in the open direction or directions according to an experimental or modified Hankel function. The particular behavior of the open resonator depends upon the dimensions and material constants of the dielectric bodies as well as the instantaneous operating frequency of the resonator. The quality factor Q of either a one or a two-dimensional open resonator is determined by the dielectric and galvanic losses of the resonator. The quality factor of the three-dimensional open resonator is determined by dielectric and radiation losses of the resonator.
Universally shielded resonators are advantageous since they make it possible to obtain especially high quality factors, if the size of the shield is at least twice the largest dimension of the dielectric resonator. With dielectric resonators, however, it is not possible to obtain quality factors which are higher than the characteristic value cot.delta.(.delta.=loss angle), of the dielectric material.
Although the dielectric resonator is fully described in the literature, one finds few practical uses for it. One reason is the relatively small intervals which exist between successive resonant frequencies. In addition, certain problems are encountered when constructing such filtered structures. Thus, to obtain low filter losses, highly loss-free dielectrics are required.
Coaxial quarter-wave resonators are especially useful in tank circuits for multiple circuit filters e.g. as bandpass filters with comb-like (combline) or finger-like (interdigitally) arranged conductor structures. The preferred frequency range of such structures is between 500 MHz and about 5 GHz, whereby it is possible to attain quality factors as high as two to three thousand.
Cavity resonators are primarily useful in circuits where low transmission losses with high selection are required. For example, cavity resonators are useful as antenna filters in highly sensitive microwave receivers. In most cases, the quality factor of such resonators lies in the range of 5,000 to 10,000. However, these resonators require a relatively large volume in the low frequency range and are therefore relatively heavy. In some instances, metalized ceramic bodies are utilized to reduce the weight of cavity resonators. Such resonators, however, are expensive and different to construct.
It has been found that regardless of the resonators utilized, high quality factors can be realized only with resonators having a large conductor surface area and/or a large cavity volume. This is a result of the isotropy of the mediums which always penetrate into the resonator cavity of the electromagnetic field.